1. Field of the Invention
The present invention relates to a spring with high durability, which is excellent in corrosion resistance and chipping resistance, and a method of coating the same.
2. Description of the Related Art
Various kinds of suspension springs have been used in motor vehicles, railroad vehicles and so on. Many of these suspension springs are made of steel, and coating adapted to impart corrosion resistance thereto has been normally applied to surfaces thereof. However, when the motor vehicles and so on are running, pebbles and gravels spattered by their wheels strike the suspension springs to cause peeling of coating films due to striking impact, that is the so-called chipping. As a result, uncoated surfaces (basis materials) of the suspension springs become exposed, and rust is formed at portions where uncoated surfaces become exposed. Accordingly, having high chipping resistance as well as providing high corrosion resistance is required to coating of the suspension springs.
On the other hand, coating composed of a plurality of layers has been applied to bodies of motor vehicles, considering corrosion resistance, chipping resistance, appearance of bodies and so on. The bodies of the motor vehicles and the suspension springs, however, differ from each other in composition, strength and so on. And, great distortions are generated in the suspension springs due to deformations thereof. Consequently, the suspension springs require a special coating exhibiting corrosion resistance and chipping resistance.
From these viewpoints, for example, U.S. Pat. No. 5,981,086 discloses the technique of imparting corrosion resistance and chipping resistance to a high-tensile steel by applying a two-layered coating including a first layer composed of a thermosetting epoxy which contains zinc with a predetermined ratio, and a second layer composed of an ethylene/acrylic copolymer. Furthermore, U.S. Pat. No. 6,663,968 discloses the attempt of adding fibers and foaming agents to a topcoat layer not including zinc in a two-layered coating for high-tensile steel.
Normally, as a measure for improving chipping resistance of a coating film, to make the coating film thick is proposed. However, when the coating film is made to thick, internal stress increases, and the coating film is easily peeled off. Furthermore, the cost increases. On the other hand, in consideration of the use in cold districts, corrosion resistance and chipping resistance at low temperatures become a big problem. However, by a method of coating disclosed in U.S. Pat. No. 5,981,086 and U.S. Pat. No. 6,663,968, a coating film having a desired performance cannot be obtained.